This invention relates to a method and apparatus for carrying out a non-linear operation on a digital signal
A video switcher may be used to combine input video signals from video sources to provide an output signal representing a picture composed partly of the image represented by one input video signal and partly of the image represented by the other input video signal The combination of the two input video signals may be accomplished by use of a mixer that selectively outputs one or the other of the video signals or a proportion of both in response to a control signal, commonly called a key signal. The control signal may be generated by performing a non-linear operation on a third video signal or on a wipe waveform or on one of the two input video signals. In the following description the term wipe is used to mean any one of these means of generating a control signal. In a wipe, the switcher receives input video signals representing two images and provides an output signal that changes from one input video signal to the other in response to a wipe signal, which defines a predetermined wipe pattern. FIG. 1 illustrates a simple split screen wipe, where the left side of the output picture is the left side of one input image and the right side of the output picture is the right side of the other input image. In this case, a wipe signal having a ramp waveform, which is a low voltage in areas corresponding to the left side of the picture and a high voltage in areas corresponding to the right side of the picture, as shown by the waveform A in FIG. 2, is generated. This wipe signal becomes the input to a clip and gain circuit (FIG. 3). The clip and gain circuit has a comparator 2, in which a clip level is subtracted from the voltage of the wipe signal, and the resulting difference signal (waveform B) is amplified by a multiplier 4 to provide an output signal (waveform C) which is limited at 8 to provide a key signal (waveform D). The key signal is applied to the control input of a mixer 10 that receives the input video signals at its two video input terminals. The key signal is indicated in FIG. 2 as having a range from -1 to +1, in arbitrary units. The output video signal may be described by EQU Video out=1/2 Video 1 (1+key)+1/2 Video 2 (1-key)
When the key signal value is 0, the video out luminance is composed of 50% of the luminance of video 1 plus 50% of the luminance of video 2, and therefore the locus of points for which the key signal value is 0 represents the boundary between the two images.
The clip level is under operator control and sweeps through a range of values as a manually operated control, such as a lever arm, is swept through a range of positions. In this way, the boundary between the two images represented by the input video signals may be moved horizontally, e.g. from left to right of the field By use of appropriate wipe signals, a boundary that is horizontal or inclined may be provided, and the boundary may be moved vertically or along an inclined path by adjusting the clip level.
A clip and gain operation may be implemented in the digital domain or the analog domain. In either case, problems can arise because the mixing operation is a multiplication process. Both the input video signals and the wipe signal have a potential bandwidth of 5 MHz, so that when these signals are multiplied together the resulting signal can have frequency components up to 10 MHz. In the analog domain, the out of band energy causes ringing in the band-limiting filters. In the digital domain, there is energy above the Nyquist frequency, which results in in-band alias frequencies.
In the digital domain, a second problem arises because the limiting that takes place in the clip and gain circuit is a non-linear process that can produce an infinite spectrum from an in-band signal. The components that are above the Nyquist frequency again produce in-band alias frequencies, which result in jaggies on key edges.
The waveforms E and F in FIG. 4 represent a digital wipe signal that is applied to a clip and gain circuit on successive lines of a video signal. The sample points are represented by circles, and the broken lines represent the analog waveform that results when the digital signal is passed through an appropriate reconstruction filter. The clip and gain circuit has a linear region, which is represented by the shaded band in FIG. 4. All wipe signal values that lie above the linear region are mapped to a key signal value of +1, all wipe signal values that lie below the linear region are mapped to a key signal value of -1, and wipe signal values that lie in the linear region are mapped linearly to key signal values between -1 and +1. The wipe signal represents a sloping boundary between the two images that form the output picture, in that the points at which the waveform of the wipe signal crosses the time axis on the two lines are spaced apart along the time axis.
FIG. 5 shows a reconstruction of a digital wipe signal on five consecutive lines of a video field. The sample values of the digital wipe signal represent a sine-squared edge having a range of +/-5 units. The sample times are represented by the vertical lines The digital key signal is limited to a range of +/-1 unit. The five sine-squared edges, occurring at different times, each result in a key edge that occurs at the same time. Ideally, five key edges, which intersect the five wipe edges respectively on the time axis, should be provided.
As shown in FIGS. 4 and 5, it is possible for all wipe signal sample values on two or more consecutive lines to be outside the linear region of the clip and gain circuit. If this occurs, the waveform of the key signal that is produced in response to the wipe signal is the same on the multiple lines. Accordingly, the desired sloping boundary between the two images is rendered as a succession of vertical segments joined by horizontal or near horizontal segments. The resulting jaggies may be visually disturbing.